The invention relates generally to a cutting insert to be placed into a tool holder for boring holes into metals. More specifically the invention relates to a cutting tool insert having a specialized geometry including a chip splitting groove formed at a negative rake angle on the cutting surface allowing increased strength and higher feed rates.
Drilling systems are frequently used to provide cylindrical holes in metallic workpieces. The cutting or boring action of the drill system may be carried out by an elongated, substantially cylindrical drilling tool, such as a combination of a tool holder and a drill insert, which is selectively attached thereto. Such an arrangement may then be used in an application wherein one end of the tool holder is securely mounted in a driving apparatus, which rotates the holder about its longitudinal axis. At the opposite end of the elongated tool holder, the cutting insert engages the material to be cut. Alternatively, the workpiece may be made to rotate relative to the holder and cutting insert, such as in positioning the holder in the tail stock of a lathe or the like. Further, the tool and workpiece may be made to rotate relative to one another. The use of cutting inserts allows for quick changing of the insert upon wear of the cutting surfaces instead of the entire tool, and allows for one tool to be used for a variety of different boring applications by simply changing the insert and not the entire drill assembly.
One problem with prior art cutting tools is that oftentimes improvements to the insert to increase cutting efficiency also reduce the insert strength. An example is the use of chipbreakers which are typically formed parallel to the clearance on the back side of the cutting edge. Referring to FIG. 1, a typical prior art spade type insert 2 is shown in a cross-sectional view taken through the center of a chipbreaker 4 formed through the cutting edge 6 of the insert 2. Behind the cutting edge 6, the primary clearance 8 drops down at an angle xcex1 from the horizontal. The clearance prevents unwanted contact between the work piece and the insert 2. The prior art insert 2 also comprises a secondary clearance 9 formed at an angle xcex8 from the horizontal, which is larger than angle xcex1, creating additional clearance. The dual angled clearance surfaces 8, 9 allow more material behind the cutting edge 6 than a clearance formed solely at the secondary clearance angle xcex8, and more clearance than that provided by the primary clearance angle xcex1. Chipbreaker 4 is shown formed at an offset distance D, parallel to primary clearance angle xcex1. Due to the secondary clearance angle xcex8, chipbreaker 4 breaks through the clearance surface of the insert 2 at point P. Although not shown, there will also be a breakout point P if the chipbreaker is not formed parallel to the clearance angle xcex1, but rather at a smaller angle down from the horizontal than clearance angle xcex1. Although also not shown, prior art chipbreakers can also extend across the entire width W of the insert 2. The removal of material across the width of the insert, or a significant portion of the width of the insert, can significantly reduce the insert strength. This is especially true with inserts made of sintered metallic hard materials which are generally more brittle than other materials. Typical sintered hard materials include carbide, cermet, ceramic, monocrystalline and polycrystalline diamond, boron nitride, etc.
The present invention provides a novel drill insert having chipbreakers formed on the cutting surfaces. The chipbreakers increase the cutting efficiency of the tool while allowing the edge strength to be increased. The drill insert of the present invention allows higher feed rates. These and other objects of the invention will be apparent as disclosed in the detailed description of the drawings below.